Iranian leaders have long said that they knew nothing about the missing American, and United States officials have said that he may no longer be in Iran — or even still alive. Aside from a hostage video and photographs of him in an orange jumpsuit five years ago, there had been no public clues about his fate.

But newly disclosed documents suggest that Iranian officials knew far more about Mr. Levinson. In late 2011, a top Iranian diplomat acknowledged that his country was holding the American and would release him if the United States helped delay an assessment criticizing Iran’s nuclear activities, the documents say.

Iran’s ambassador to France at the time, Seyed Mehdi Miraboutalebi, made the statement during a private gathering at his Paris residence with two men working with an American religious organization, according to a report about the session. The meeting resulted from a letter sent to Ayatollah Ali Khamenei, Iran’s supreme leader, by a leader of the religious group, the Fellowship Foundation, which had previously helped win the release of an Iranian-American imprisoned in Tehran.

“The ambassador made it clear that they have Robert Levinson and that they are willing to release him without conditions,” said the report, which was sent to the F.B.I. in October 2011. “They do, however, want tangible, ‘symbolic’ assurances that the messages they are sending have been received at the highest levels.”

Iranian officials maintained that they did not know Mr. Levinson’s whereabouts or status during recent negotiations that resulted in the prisoner exchange last week. The Obama administration has never challenged Iran’s position, but law enforcement officials say they believe that factions tied to that country’s intelligence, political or religious leadership were involved in his capture and detention.

It is unclear what actions, if any, American officials took after the ambassador’s remarks in Paris. Mr. Levinson’s wife, Christine, said in an interview Friday that she was never told about the report of the Paris meeting, and wondered why government officials did not use the information to help her husband.

“If this happened in 2011, then why isn’t Bob home by now?” she asked.

Soon after the episode in Paris, a law professor at Catholic University of America in Washington affiliated with foundation, Robert A. Destro, sent a detailed report about it to the F.B.I. Four F.B.I. agents also interviewed an American living in Paris, one of the two men who met with the diplomat.

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An excerpt from a 15-page memo sent to the F.B.I. in 2011 after a meeting in Paris between an American, a Russian and Iran’s ambassador to France in which the ambassador said his government was willing to release Robert A. Levinson, who was being held by Iran.

An F.B.I. spokeswoman, Lindsay Ram, declined to comment, and a State Department spokesman, Samuel Werberg, said that officials could not comment about the Paris meeting because they were not involved in it. He did not respond when asked if the State Department had received Mr. Destro’s report.

Mr. Miraboutalebi could not be reached for comment, and an Iranian government spokesman in New York issued a statement echoing remarks this week by a White House spokesman that the Obama administration did not believe that Mr. Levinson was in Iran. The Iranian spokesman added that his government had offered cooperation, on a humanitarian basis, “to help determine his whereabouts.”

The assurances the Iranian diplomat sought in late 2011 in return for Mr. Levinson’s release involved assistance from the United States in delaying a report about to be released by the United Nations nuclear watchdog, the International Atomic Energy Agency. The I.A.E.A. had concluded that Iran had used its nuclear energy program, which it claimed was for peaceful purposes, to try to develop weapons.

The events surrounding the Paris meeting and documents containing Mr. Miraboutalebi’s statements are drawn from a forthcoming book about Mr. Levinson and the search for him.

A former F.B.I. agent who became a private investigator, Mr. Levinson, then 59, disappeared on Kish Island, an Iranian island in the Persian Gulf, while trying to recruit a fugitive American-born assassin as an intelligence source inside Iran. He was last seen alive in the hostage videotape made in 2010 that did not disclose who was holding him.

C.I.A. officials have said that Mr. Levinson, who had a contract with the spy agency’s analytical unit, went to Kish on a “rogue” unapproved mission and that they would have tried to stop him had they known about it.

After his disappearance, Mr. Levinson’s family and friends were frustrated by what they viewed as the United States government’s lackluster efforts to find him. But even after the F.B.I. committed more resources to the hunt, bureau officials faced difficulties in penetrating Iran for intelligence about the missing American.

As a result, they turned to businessmen and others willing to aid the search in exchange for favors from the United States, such as obtaining visas or having their names removed from watch lists. Those recruited that way included Boris Birshtein, a Toronto-based businessman, and Madzhit Mamoyan, an ethnic Kurd who lives in Moscow.

By 2011, the F.B.I. had severed ties to the men because bureau officials believed they had not produced worthwhile information. But in the summer of that year, Mr. Birshtein went to see Douglas Coe, a lay minister and spiritual leader of the Fellowship Foundation, about Mr. Levinson. The businessman considered Mr. Coe a friend and religious mentor.

The Fellowship, a Christian group whose ranks include American politicians and business leaders, had worked to help win the 2007 release of an Iranian-American scholar, Haleh Esfandiari, from Tehran’s Evin prison.

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Robert Levinson in an undated photograph released by his family. A C.I.A. consultant, he had also worked for the F.B.I.CreditChristine Levinson, via Associated Press

Mr. Coe had also been part of an American religious delegation that went to Iran in 2003. After Mr. Birshtein’s visit, he contacted Mr. Destro in mid-2011 and asked the law professor to write a letter to Ayatollah Khamenei. It was delivered to the Iranian ambassador in Paris.

The diplomat responded by requesting an urgent meeting in late October. The ambassador was days away from ending his posting to France and returning to Tehran. Mr. Destro could not travel on short notice, so Ory Eshel, an American businessman living in Paris, represented the foundation at the meeting.

Mr. Destro also alerted the F.B.I. about the foundation’s involvement in Mr. Levinson’s case and its interactions with religious leaders in Iran. “We wanted to be upfront and clear with them,” he said.

On Oct. 30, 2011, Mr. Eshel and Mr. Mamoyan arrived at the Iranian ambassador’s residence, according to the documents.

Initially, the diplomat spoke at length about the deep distrust between his country and the United States, according to the reports of the session. He then brought up the atomic agency report, expected to be released in mid-November. Mr. Eshel noted in his account that the diplomat was apparently aware of its findings, attacking them as “fake” and adding that he believed that American officials had pressured inspectors to adopt them.

“The ambassador emphasized that the I.A.E.A. report would be devastating,” and set off a chain of events “that would back both sides into a corner,” Mr. Destro’s report states.

Mr. Eshel and Mr. Mamoyan then asked about Mr. Levinson. The Iranian diplomat stated that he was in Iran, Mr. Eshel said. He added, “There was absolutely no shadow of a doubt” about the diplomat’s meaning. Mr. Miraboutalebi did not respond to inquiries about Mr. Levinson’s specific location or which faction in Iran was holding him, Mr. Eshel added.

The ambassador said he planned to meet with Ayatollah Khamenei upon his return to Iran and intended to say that religious groups like the Fellowship Foundation could help “reset” relations between Washington and Tehran. One way to demonstrate that influence, he told his two visitors, was to have American officials agree to briefly delay the I.A.E.A. report’s release.

Later that day, Mr. Mamoyan was called back to the ambassador’s residence. According to the report given to the F.B.I., the diplomat told him that Iran wanted to discuss “several Iranians held by the United States” and that Mr. Levinson would be set free if a deal on the nuclear report could be reached by its anticipated release date of Nov. 17.

The I.A.E.A. report came out on Nov. 8, a week after the meeting.

Correction:

Because of an editing error, an earlier version of a caption for a document image with this article misstated the nationality of one of the two people who met with Iran’s ambassador to France. One was a Russian and the other an American; they were not both Americans.

Thomas Erdbrink contributed reporting from Tehran.

This article is adapted from “Missing Man: The American Spy Who Vanished in Iran” by Barry Meier, to be published May 3 by Farrar, Straus & Giroux.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Clues Emerge on American Sought in Iran. Order Reprints | Today’s Paper | Subscribe